Some modern telephone intercom systems interconnect with standard key telephone systems to provide both intercom and basic telephone service from the same station sets. The intercom systems provide a private communication network having a variety of telephone call features. In one type of intercom system these telephone call features are initiated by entering a two digit code from the rotary or multifrequency dial of the connected station set.
Additional call features are continually being added to enhance the telephone service and convenience of intercom systems. One such calling feature is called a Do-Not-Disturb feature. The Do-Not-Disturb feature enables a station to appear busy to incoming intercom calls. The desirability of more flexible call features had led to a selective Do-Not-Disturb feature which selectively blocks calls to a station.
In the prior art arrangements selective Do-Not-Disturb features are referred to as call screening or call restriction arrangements. The prior art call screening arrangements have selectively screened calls on a station, class-of-service or user basis. For example, in one arrangement the operation of a single key at the attendant's PBX console enables a class-of-service call restriction circuit. This class-of-service circuit results in specific limited telephone service to a particular group of telephones served by the PBX. In another arrangement a class-of-service call restriction system not only prohibits calls but determines the alternative destination of the attempted prohibited calls.
In still another system a call screening circuit blocks all calls to the called station unless the calling station provides a predetermined signaling code which inhibits the screening circuit. Thus, only those telephone calls in which the calling party initiates appropriate code signals, in addition to the called station's telephone number, will actuate the audible means to the called station. This circuit provides call screening on a "calling party" basis rather than a "station" basis. Hence, only calling parties knowing the called station selected signaling code can bypass the screening function and ring the called station. In addition, the selected code can be altered as often as desired by the called station.
In some situations the prior art selective call screening arrangements may be too static and not flexible enough. A telephone user may, for example, desire to screen incoming telephone calls according to the "type" of call received rather than on a calling party, class-of-service or station basis. Thus, the type of call screening could be determined by the particular features activated at a called station. For example, a telephone user may spontaneously desire to exclusively communicate, to the exclusion of all other incoming telephone calls, with a certain telephone location which is currently busy. Hence, a selective call screen must be activated by the user to inhibit the reception of all incoming calls except for the call back from the busy station. In the above example only the busy station can "callback" and selectively bypass the call screening feature and signal the called station. Thus, it would be desirable for a selective call screening arrangement to provide various dynamic selective call screening capabilities.
Accordingly, it is a problem to design a call screening arrangement which screens calls according to the type of call received at the called station.
It is an additional problem to selectively screen calls according to the particular call features activated at the called station.
It is a further problem to design additional call screening features into an intercom system without requiring the user to memorize the feature activation codes.